1. Field of the Invention
The present invention relates to a power semiconductor device and a method for manufacturing the same. More particularly, the invention relates to a power semiconductor device in which ions in a predetermined area or areas of either the collector region or the buffer region or both are selectively activated by a laser annealing technique to improve characteristics of the power semiconductor device, such as short circuit capacity, and also relates to a method for manufacturing such a power semiconductor device.
2. Background Art
Power semiconductor devices have been widely used both in industrial appliances such as inverters and in household appliances such as microwave ovens. Especially, IGBTs (Insulated Gate Bipolar Transistors) have become popular as power electronic devices, since they have a low on-state voltage and provide increased electrical strength. An IGBT includes an emitter region of a first conductivity type and a base region of a second conductivity type in contact with each other. The base region constitutes a portion of the MOS structure and includes a region whose conductivity type can be reversed by applying a voltage to the gate of the MOS structure. An electrical strength providing region of the first conductivity type is provided in contact with the base region. When the IGBT is turned on, the resistance of the electrical strength providing region is reduced by conductivity modulation, resulting in a reduced on-state voltage. Further, a collector of the second conductivity type is provided in contact with the electrical strength providing region, and a guard ring structure (a loop-like structure) of the second conductivity type is provided along the periphery of the electrical strength providing region. The guard ring structure contributes to reducing the electric field in the electrical strength providing region.
When the IGBT is turned off, if an excessive number of carriers of the second conductivity type are present in the electrical strength providing region, it may result in degraded turn-off switching characteristics. Furthermore, the parasitic thyristor formed by the emitter region, the base region, the electrical strength providing region, and the collector region may turn on, resulting in degraded controllability of the current of the device.
Japanese Laid-Open Patent Publication No. 2003-133556 discloses a method for manufacturing an IGBT in which the collector region is formed only on the back side of the active region, which does not include guard rings. That is, the collector region does not extend directly below the guard rings. This avoids a situation where carriers of the second conductivity type are injected from the collector region into the electrical strength providing region in a quantity greater than an appropriate level when the IGBT is turned on, thereby preventing the problems described above. Other related art includes Japanese Laid-Open Patent Publication Nos. 2005-333055 and 2006-059876.
The IGBT and the manufacturing method therefor disclosed in the above Japanese Laid-Open Patent Publication No. 2003-133556 are adapted to prevent the problems of degraded turn-off switching characteristics of the IGBT and the turn-on of the parasitic thyristor. However, this IGBT is disadvantageous in that the collector region is not fully in ohmic contact with the collector electrode, since the collector region is formed only in the active region, that is, the collector region does not extend directly under the guard rings.